Equine Viral Arteritis (EVA) is a contagious disease caused by the equine arteritis virus (EAV). The virus occurs worldwide including mainland Europe.

The disease is notifiable: if you suspect the disease, you must immediately notify the duty vet in your local Animal Health Veterinary Laboratories Agency (AHVLA) office.

Latest situation

In recent years, equine viral arteritis (EVA) has been confirmed in (i) a stallion in Staffordshire on 2 August 2010, (ii) a stallion in West Sussex on 8 December 2010 and (iii) a stallion in Gloucestershire on 4 October 2012. These cases are now closed.

Defra monitors the occurrence of major animal disease outbreaks worldwide as an early warning to assess the risk these events may pose to the UK. International disease monitoring assessments are available.

About the disease

The variety and severity of clinical signs of EVA vary widely. Infection may be obvious or there may be no signs at all. Even when there are no signs, infection can still be transmitted and stallions might still become shedders.

EVA can cause abortions. Other signs include fever, depression, lethargy, stiff movement, runny nose, conjunctivitis, (‘pink eye’), swelling of the lower parts of the legs, around the eye and of the reproductive organs.

Because of the variability or the possible absence of symptoms, clinical diagnosis is not always possible. Laboratory diagnosis is therefore essential. This requires appropriate samples to be taken and sent to a specialist laboratory.

How is EVA treated?

There is no treatment available for EVA itself, although there may be treatments to alleviate some of its symptoms. These should be determined by the attending vet.

Spread/transmission

Infection spreads through transmission of the virus between horses in 4 main ways:

• venereal infection of mares by stallions during mating
• artificially inseminating mares with semen from infectious stallions
• contact with aborted foetuses and other products of parturition
• direct contact in droplets from the respiratory tract (e.g. through coughing and snorting).

The Shedder Stallion

Stallions can pose a significant risk of disease transmission if undetected. On infection, the virus localises in his accessory sex glands and the virus may be “shed” i.e. excreted it in his semen, for several weeks afterwards, or for many months or years, and possibly for life. After recovery from acute illness, his fertility is not affected and he will show no further clinical signs of infection even though he may still be infectious. Shedder stallions will infect susceptible mares during mating, or after insemination with the stallion’s semen, and these mares may, in turn, infect in-contact animals through either mating or via the respiratory route.

It is important to note that the shedder stallion is always seropositive (ie past or existing infection indicated in a blood test) but that a seropositive stallion is not necessarily a shedder.

Breeders using artificial insemination (AI) must note that the virus can survive in chilled and frozen semen.

Teasers

Teasers are also a potential source of the virus and should be subjected to the same precautions as stallions.

Mares

Available evidence indicates that the ‘carrier’ state does not occur in mares, i.e. mares can become infected, and be infectious, but they are not believed to remain infectious for a long time, either continuously or intermittently.

Disease prevention

The Code of Practice produced by the Horserace Betting Levy Board sets out recommendations for horse breeders for the prevention and control of diseases during the breeding season. Biosecurity is an important part of good management and owners should not breed from horses or use semen of unknown EVA and Contagious Equine Metritis status.

It also sets out measures to control infected horses. Any occurrence of this disease in the UK will be controlled in line with the industry-agreed HBLB Code of Practice.

EVA Vaccine

A licensed vaccine (Artervac, Fort Dodge) is available in the UK and Europe, but with different national restrictions on use. In the UK, the vaccine can be used in all horses and ponies over nine months of age. It should be used as per the datasheet, which recommends booster vaccination every six months.

It is important to note that vaccinated horses will become seropositive (have a positive test result for the presence of the antibody in the serum of the blood) and this cannot be distinguished from true infection. We suggest discussing vaccination with your vet to obtain advice that is relevant to the use of your horse.

Horses should be blood tested before vaccination to show that they are likely to be free of infection at the time of vaccination, and vaccination must be maintained unbroken every six months thereafter; otherwise additional laboratory testing will be needed to distinguish positive test results that are due to vaccination from those that may be due to infection. Details about the prior blood test and the vaccination, both primary and booster, should be recorded in the horse’s passport.

Records are important as evidence of previous seronegativity for breeding and export purposes. Some importing countries require this information for vaccinated horses, either in passports or official export certificates.

As the vaccine’s efficacy is not yet fully known, owners should monitor the horse’s antibody response after vaccination, in consultation with the vet.

Legislation

In the UK, Equine Viral Arteritis (EVA) is a disease which is notifiable by law under The Equine Viral Arteritis Order 1995 made under section 88 of the Animal Health Act 1981.

Under the Order, anyone who owns, manages, inspects or examines a horse must notify their local AHVLA office when:

• they suspect the disease in a stallion, either on the basis of clinical signs or following blood or semen testing;
• they suspect disease, either on the basis of clinical signs or following blood testing, in a stallion or mare that has been mated or artificially inseminated within the past 14 days.

Under the Order, Defra may:

• serve notices prohibiting the use for breeding of the suspect stallion and any semen obtained from it unless permitted under licence by a veterinary inspector, or an inspector acting under the direction of a veterinary inspector;
• take samples or obtain information in order to establish whether disease is present and, if so, the extent to which it has spread.